<template>
    <p><input type="text" v-bind:placeholder="username"></p>
    <p><input type="password" v-bind:placeholder="password"></p>
</template>

<script setup>
       import {ref} from 'vue'
       const username = ref('请输入用户名')
       const password = ref('请输入密码')
       setTimeout(() => {
        username.value = '请输入用户名', password.value = '请输入密码'
       })
</script>